Submitted to: Chemoecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/4/2002
Publication Date: 9/3/2002
Interpretive Summary: Natural plant-produced chemicals affect insect herbivores in many ways, and understanding how these components affect pest behavior, distribution, survival, or development may have pest management implications. In this study we determined how the distribution of nicotine within flue-cured tobacco plants affected the distribution of tobacco hornworm and tomato hornworm larvae, which are major pests of tobacco, tomato, and other solanaceous plants. Larvae of these two species differed in where they fed within tobacco leaves. Tobacco hornworm larvae fed closer to the base of the leaf, whereas larvae of the tomato hornworm fed more toward the tip of the tobacco leaf. Nicotine concentrations varied within each leaf, increasing 2-3 fold from the base to the tip of the leaf. Nicotine concentrations also increased 7-10 fold from the top to the bottom of tobacco plants, but both hornworm species selected leaves in the middle region of the plant. The distribution of these hornworm species within leaves may be partially explained by their differential responses to nicotine as determined by laboratory experiments. In these tests, both species responded to nicotine as a feeding deterrent; however, the response by tomato hornworm larvae was less rigorous. Parasitism of tobacco hornworm larvae was also affected by where the larvae were feeding on the leaf. These results suggest that hornworm larvae can detect nicotine within tobacco leaves, which affects their selection of feeding sites. Knowledge of larval feeding sites may affect considerations for chemical applications, biological control, or other pest management options.
Technical Abstract: Third-instar larvae of Manduca sexta L. and Manduca quinquemaculata (Haworth) (Lepidoptera: Sphingidae) differed in their within-leaf feeding site selections on tobacco in the field. Manduca sexta fed more proximally and centrally on the leaf; whereas, M. quinquemaculata fed more distally. Within-plant selection of leaves did not differ; both species selected leaves in the middle region of the plant. Nicotine concentrations varied within each leaf, increasing 2-3 fold from the proximal to distal area of the leaf. Nicotine concentrations varied within each plant, increasing 7- 10 fold from the first fully expanded leaf to the twelfth (lowest) leaf. In laboratory bioassays, both species responded to nicotine as a feeding deterrent. Electrophysiological studies demonstrated that gustatory organs of both species responded to nicotine at concentrations found in tobacco leaves. Field mortality of M. sexta due to parasitism by Cotesia congregata and to parasitism and predation combined differed among feeding sites; predation alone did not.